1. Field of the Invention
The present invention relates to a frequency sensor and, more particularly, to a card-shaped or similar frequency sensor contrived to be attached to an article such as a video tape, video disc, compact disc or audio disc for the purpose of preventing theft thereof.
2. Description of the Prior Art:
Regarding the conventional frequency sensors of the type mentioned, there is known an example disclosed in Japanese Patent Laid-open No. 61 (1986)-149880.
For achieving a dimensional reduction in such known frequency sensor, a transmitting antenna pattern is disposed in a U-shaped receiving antenna pattern, and an anode of a multiplier diode is connected to a predetermined portion of the receiving antenna pattern, while a cathode of such diode is connected to a predetermined portion of the transmitting antenna pattern.
The above frequency sensor is considered particularly effective as a type to be suspended from a garment, gem or the like for prevention of theft.